For a magnetic recording medium, a higher coercive force and a much thinner recording layer are necessary to increase recording density upon magnetic recording. In a so-called coated type magnetic recording medium which is prepared by mixing, kneading and dispersing magnetic particles with a binder and coating the thus obtained dispersion on a non-magnetic support and drying, reduction in the thickness of a recording layer is limited because it contains binders. On the other hand, in a so-called thin ferromagnetic metal film type magnetic recording medium which is prepared by providing on a support a thin ferromagnetic metal film by physical vapour deposition method (such as vacuum evaporation or sputtering, chemical phase growing method, or a metal plating method such as an electroless plating method or electric plating), it is possible to provide a recording layer having a thickness of 0.1 micron or lower.
However, when a medium's recording layer is as thin as 0.1 .mu.m or lower, the medium is likely to be influenced largely by the nature of the surface of the support. Desirably, the surface of a support should be smoothed. However, the friction coefficient of the medium is increased and running properties upon recording and reproduction are remarkably decreased by smoothing the surface.
Many approaches have been proposed to solve the above problems of providing a smooth surface without increasing the friction coefficient and decreasing the running properties upon recording and reproduction.
It is disclosed in various patents such as U.S. Pat. Nos. 4,152,469, 4,333,985 and 4,390,601 that an overcoat layer composed of organic compounds is provided on a thin ferromagnetic metal film. However, in such media there is a problem that organic compounds of the overcoat layer rapidly come off from the thin metal film after repeated use. These organic compounds then attach on a head or a guide pole, thereby causing the tape to stop running.
It is also disclosed in U.S. Pat. No. 4,540,618, JP-A-58-100221 and U.S. Pat. No. 4,578,729 that microscopic projections are formed on a surface of a thin ferromagnetic metal film. (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, in such media, ferromagnetic metal particles which have abnormally grown from inorganic fine particles as a nucleus present on a support are used as microscopic projections, and this use has the undesirable effect of causing an increase of noise.
It is disclosed in JP-A-59-32580, and JP-A-62-130848, and in U.S. Pat. Nos. 4,670,319 and 4,508,182 that fine particles are incorporated into a plastic film to form unevenness (concave & convex) on the surface of a film of a non-magnetic support. In such media, the height and the shape of the unevenness (concave & convex) vary depending upon the position of the particles in the thickness direction of the film. When the height of unevenness is high and the uneveness is steep, these variations cause drop out, and when the height of unevenness is low, sufficient running properties cannot be obtained. Further, when the direction and the shape of unevenness (concave & convex) are irregular, the envelope of reproduced output is disturbed.
On the other hand, JP-A-58-68223 and JP-A-58-68224 disclose a flexible support having microscopic unevenness, which is prepared by coating silicon emulsion on a support and then stretching. However, the above undercoating is likely to separate at the stretching step, and thus partially cause deterioration of the adhesiveness between the undercoating and the support, thereby causing drop outs in many cases.